This invention relates to a method of burnishing malleable films on semiconductor substrates and more specifically relates to a method of burnishing bonding pads of gold or gold alloy on semiconductor devices.
It is known to use thermocompression bonding to bond semiconductor devices to headers and heatsinks in the semiconductor art. Often, to facilitate thermocompression bonding, thick metallic films, called bonding pads, are deposited on the surfaces to be joined. The bonding pads are typically of a malleable metal or alloy. When the two bodies are pressed together, the bonding pads compress and are fused by the appropriate combination of temperature, pressure and time. The malleable nature of the pad can also serve to absorb some of the stress of the pressure applied during the process. Thermocompression bonding provides bonding of two objects at a temperature lower than the melting point of the pad material and without the use of a flux.
A problem with bonding small semiconductor devices to heatsinks is the susceptibility of the device to damage or breakage from the pressure applied. For example, electroluminescent devices of gallium arsenide, indium phosphide and the like, because of their inherent strain, can only withstand limited amounts of stress before crystalline damage occurs. Often the pressures required to bond these devices to heatsinks using high-quality gold of gold alloy bonding pads approach the stress limits for the crystalline materials. This damage adversely affects the life of the device.
Even when the bonding is carried out at pressures below the stress limits for a given crystal, damage can result. Misalignment of the device and the heatsink can concentrate all of the pressure in one small area and this point pressure can far exceed the stress limits for the crystalline device. Excessive pressure is also exerted on nodules or high points on the bonding pads.
It therefore becomes important to apply a uniform, parallel pressure to the bonding pads during the bonding process. Providing bonding pads with smooth, defect-free surfaces is the first step towards accomplishing this purpose.
U.S. Pat. No. 3,676,214 to English et al. describes a method which substantially eliminates defects, such as pinholes, in a gold film. The method employs burnishing, which does not scrape or abrade the surface but rather compresses and planarizes the surface to produce a smooth mirror-like finish on the malleable film. The burnishing process can be compared to the effect that a rolling pin has on dough, i.e., the material is leveled by moving it around rather than by removing it. For 5000-40,000 Angstom (hereinafter .ANG.) thick gold films, English et al. suggest vibratory ball burnishing (the vibration of several layers of steel balls on the gold surface) and wire brush burnishing.
Although the bonding pads on semiconductor devices are typically in the 5000-40,000 .ANG. thickness range, the above burnishing methods are not suitable. A typical device is only about 7.times.12 mils with a bonding pad about the same size thereon. The very size of the device precludes the use of the above burnishing methods. For example, prior art burnishing requires steel balls at least 0.1 millimeter in diameter or about 4 mils. This would provide that only about two balls could fit on the bonding pad surface, and certainly no burnishing would result. The above mentioned methods would also introduce unwanted stress and damage to the fragile semiconductor substrate.
It would be desirable, therefore, to have a method of burnishing bonding pads on semiconductor devices to provide smooth, defect-free surfaces without damaging fragile semiconductor substrates.